Carach Angren or Isenmouthe was a pass in the northwest of Mordor. The pass was formed where spurs reaching out from the ranges of the Ephel Dúath and the Ered Lithui met, leaving only a narrow passage between the Plateau of Gorgoroth and the smaller valley of Udûn to the north.[1]

As the passage to the Black Gate of the Morannon, Carach Angren was heavily fortified, and both the rocky spurs that overlooked it carried fortresses and watchtowers. Across the passage itself, a wall of earth had been built, and a great ditch had been dug across the opening spanned by a single bridge.[2]